Debris in the oil will reduce the life and efficiency of a hydraulic drive. The pump components are manufactured to extremely small tolerances, measured in microns. Any foreign matter, particularly of a metallic nature, is likely to cause damage. A filter to a minimum standard of ISO BSI/5/15 level 17/11 must be fitted, and the element changed in accordance with a maintenance programme or when the clogging indicator moves into the danger zone.
Water in the oil reservoir will cause the same problems as it does in a fuel tank, and the tank breather must be fitted with a filter.
Finally, low oil level can lead to pump failure. Any air bubbles that get into the system can implode inside the pump when compressed to the working pressure. A level gauge should be fitted.